2020 Summary – Second Warmest Year on Record

The year 2020 ended as the second warmest on record with a 24-hour adjusted mean temperature of 51.3F, which was 3.7F warmer than the 120-year (1891-2010) long-term mean and 2.4F above the 30-year (1981-2010) normal. The only warmer year was 2012 with a mean of 51.7F. The annual average maximum temperature of 60.4F was 3.5F warmer than the long-term mean, which was the fourth warmest on record. The annual average minimum temperature of 43.4F was 3.6F warmer than the long-term mean, and it was the second warmest on record.

The year started with generally very warm temperatures and below average snowfall in January and February, and those two months combined were the second warmest on record. Two exceptionally warm days in mid-January included a maximum of 65F on the 11th and the highest temperature for the month of 68F on the 12th. The latter tied the warmest temperature ever observed in January previously set on January 26th, 1950, and it surpassed the previous record for the date of 60F set in 2018. The very few cold days early in 2020 included the coldest temperature of the year, which was 4F on February 15th. The warmth continued into March, and the high of 72F on March 9th was only the second instance of a reading that high so early in the season. The weather switched to the other extreme in April, which was very cool and wet. The warmest temperature all month was 63F on April 7th, which was the coldest monthly maximum temperature in April for the entire period of record since 1885. After a near average May, the warmth returned during summer including the ninth warmest July and the sixth warmest August on record. There were two heat waves during summer 2020. The first, on July 27th-29th included the warmest temperature of the year of 95F and a minimum of 74F on July 28th. The second heat wave lasted four days from August 9th-12th, including a high of 94F on August 10th and a low temperature of 74F on the 12th. September and October were slightly warm, though October ended very cool with some early snowfall. Warm temperatures returned in early November with a period of very warm weather from November 5th-12th, including six consecutive days in the low to middle 70’s five of which set new daily record highs. Variable temperatures were the rule during December, with frequent changes between warm and cold. Christmas Day was very wet and one of the warmest on record with a high of 61F and a mean of 49F on that day.

Due to the very warm start to the year, Ponkapoag Pond was considered free of ice on February 24th, about three weeks earlier than the long-term mean date, and Houghton’s Pond thawed on the same day. The first ripe blueberries were observed on the summit of Great Blue Hill on June 23rd, which was a couple of days later than the average date. Late in the year, Ponkapoag Pond froze over on December 18th and Houghton’s Pond followed the next day, before both ponds partly thawed again later in December.

Despite a dry late summer and early fall that caused a period of moderate drought, annual precipitation was above average and totaled 53.72 inches, which was more than five inches wetter than the long-term mean but only slightly more than the 30-year normal. The highest precipitation totals occurred in April, which was the seventh wettest on record with 7.71 inches, and June, which was the tenth wettest with 7.56 inches. The driest period of the year from July to September saw only 7.07 inches of rain over those three months that would normally see 11.66 inches. This period brought moderate to severe drought to the area that was ended by a wet October and December. There were several significant storms during the year after a relatively quiet start to 2020. These included a two-day storm that brought 2.77 inches of rain on April 2nd-4th, and a daily record 4.3 inches of snowfall on April 18th, which was also the fourth largest snowfall ever observed so late in the season. Strong thunderstorms on June 28th and 29th brought a total of 4.73 inches of rain, including 1.53 inches in a single hour on the afternoon of June 28th. Thunderstorms on August 23rd brought 2.21 inches of heavy rain, which included 1.36 inches in one hour and hail up to a half-inch in diameter. A complex dual storm in late October that included the passing remnants of Hurricane Zeta, followed by a colder coastal storm, brought a total of 2.20 inches of precipitation. This event included steady rain on October 29th into the early morning hours of the 30th, which changed to heavy, wet snow that amounted to a daily record 6.5 inches of snow on October 30th that was also the second largest snowfall ever observed in October. A storm on November 30th to December 1st brought a total of 1.96 inches of rain and many hours of very strong winds that peaked at more than 80 mph with several hours averaging more than 40 mph on November 30th. The largest snowstorm of the year arrived on December 16th-17th and brought 16.5 inches, including a daily record of 15.1 inches on the 17th. The snowiest month of the year was December with 19.7 inches, and the greatest snow depth on the ground during the year was 14 inches on December 17th. The total snowfall for the calendar year was 38.1 inches, which was about two feet less than the long-term average.

The mean station pressure during 2020 at the elevation of the mercury barometer at the Observatory was 29.32 inches, which was 0.03 inches higher than the long-term annual mean of 29.29 inches. The maximum sea-level pressure during the year was 30.75 inches on January 9th. The lowest sea-level pressure during 2020 was 28.68 inches on February 7th during the passage of a cold front and its associated inland storm that also caused a peak wind gust of 71 mph from the west-southwest. This reading was the fifth lowest pressure ever observed in February since 1885, and it was the lowest pressure recorded at the Observatory in any month since 28.67 inches was observed on February 4th, 1995. 

Water vapor pressure, which is derived from the dew point and is an absolute measure of the amount of moisture in the air in terms of the pressure that water vapor contributes to the total surface pressure, averaged 10.8 mb (0.319 inches) during 2020. This value is the fourth highest on record and continues a trend of higher vapor pressures in recent decades, which is consistent with the increasing trend in temperature. Another way to interpret this value is to state that water vapor on average comprised a little over one percent of the total annual mean station pressure during the year.

The annual mean wind speed of 12.0 mph was the second lowest on record, behind only the 11.8 mph mean observed in 2012. This continues the several decade long trend of decreasing mean wind speeds on Blue Hill, though this trend has slowed somewhat in recent years. However, there were five wind gusts that exceeded 70 mph, more than in recent years, including gusts to 79 mph from the south on April 13th and 76 mph from the south on October 7th. The peak wind gust during 2020 was 81 mph from the south-southeast during a warm, windy rain storm on November 30th, and this gust was the highest at Blue Hill since 85 mph was recorded on January 18th, 2006.

Bright sunshine for the year was 53 percent of possible, which was one percent more than the long-term average. The least sunny months, April and December, were also among the wettest and had, respectively 36 and 41 percent of the possible sunshine. The sunniest months of the year were May, June and August, which had respectively 62, 61, and 62 percent of the possible sunshine.

BHO Warmest Annual 24-Hour Adjusted Mean Temperature, deg F (1885-2020):

1) 51.7 in 2012
2) 51.3 in 2020
3) 51.2 in 2016
4) 51.0 in 2010
5) 50.9 in 1999
6) 50.8 in 1998
7) 50.6 in 2006
8) 50.5 in 1953

BHO Warmest Annual Maximum Temperature, deg F (1885-2020):

1) 60.8 in 2012
2) 60.6 in 2016
3) 60.5 in 1949
4) 60.4 in 2020
5) 60.2 in 1999

BHO Warmest Annual Minimum Temperature, deg F (1885-2020):

1) 44.0 in 2012
2) 43.4 in 2020
3) 43.2 in 1998
4) 43.1 in 2006
5) 43.0 in 2010

BHO Highest Annual Mean Water Vapor Pressure, mb (1932-2020):

1) 11.0 in 2012
   11.0 in 2018
3) 10.9 in 2011
4) 10.8 in 2020
5) 10.7 in 1973

BHO Lowest Annual Mean Wind Speed, mph (1885-2020):

1) 11.8 in 2012
2) 12.0 in 2020
3) 12.1 in 2018
   12.1 in 2019
5) 12.2 in 2013
   12.2 in 2015
   12.2 in 2017

Mike Iacono
Chief Scientist
Blue Hill Observatory